When a wind turbine is not generating power, such as before connection to the grid or during times of maintenance, the blades of the turbine are to as great an extent as possible turned out the wind by means of the yaw drive and pitch drives to minimise the forces experienced from the incident wind. However, when at standstill the blades of the wind turbine are potentially prone to edgewise blade oscillations (edgewise being the direction from the leading edge to the trailing edge of the blade) resulting from the air flow across or around the blade, when the wind hits the blade from the side.
The problem is caused by the largely unstable flow of air around the rotor blade when the blade is pitched out of the wind. The air flows around the curved surface of the blade, but cannot stay attached to the blade surface in laminar flow because the blade is not pitched correctly to support lift. At high angles of attack stall is observed, with associated negative aerodynamic damping, resulting in instability of flow which can give rise to blade oscillations.
If the wind turbine blades can be feathered with respect to the wind, then the wind flows around the blades smoothly, but without power to continuously adjust the blade pitch, there will be periods when the wind direction will inevitably change and result in a situation where the flow around the blade is not smooth. At higher angles of attack a vortex-shedding phenomena can occur where vortices form on edges of the blade and are shed in a rhythmic pattern. These oscillations are typically experienced first at the tip of the blade, where the reduced diameter and lighter frame offer less resistance to the rhythmic shifting force provided by the air. The worst case is when the incident wind meets the blade perpendicular to one of its flat surfaces and has to flow around the leading and trailing edge of the blade. In this situation the vortices can be shed in a manner which creates significant vibration, particularly if resonance phenomena are exhibited, even to the extent that blade damage can result.
Many wind turbines therefore comprise mechanical or hydraulic devices that dampen oscillations induced in the wind turbine blade before they can develop a magnitude that is sufficient to damage the blade. Although, such devices do solve the problem, they can be expensive and difficult to install. It has been appreciated that there is a need for an apparatus and method for addressing such oscillations in the blade.
The Applicant has previously disclosed in EP2507513, the contents of which are hereby incorporated by reference, the use of a tool for fitting over blades at standstill in the form of a sleeve-like device formed of a net-like material having an open mesh. The mesh material of the sleeve is able to disrupt the smooth flow of air over the blade by forming a turbulence-inducing non-aerodynamic surface which thereby significantly reduces the instability associated with stall generation.
The present invention is a development of this tool directed particularly at reducing the effect of vortex-induced vibration.